1. Field of the Invention
The present invention generally relates to the manufacture of modular electronic circuits and, more particularly, the automated manufacture of multi-layer modules (MLM), especially multi-layer ceramic modules (MLC).
2. Description of the Prior Art
In electronic circuits and high-speed logic circuitry, in particular, reduction in size and increase in integration and packaging density has yielded substantial performance improvements due to reduced signal propagation time over shorter paths and increased noise immunity due to reduced capacitive and inductive coupling to conductors and other effects. For this purpose, modular circuits have been in use for several years to allow connection of numerous integrated circuit chips of diverse types and formed by different and potentially incompatible technologies within a single, compact package. Modular circuits also provide advantages of improved heat dissipation, regulation of temperature among chips, stability and protection for complex connection arrangements which are embedded therein and the possibility of electrostatic shielding being incorporated into the design.
Modular circuits are formed by a plurality of lamina, each having a conductive pattern formed on one or both sides and perforations, known as vias, filled with conductive material for making connections between lamina or sides thereof. Generally, the conductive patterns and filling of vias is performed by screening of conductive paste through a mask using specially designed machinery. Each conductive pattern will generally be unique for each lamina of a modular circuit. The lamina are individually placed in a stack and carefully aligned with previously placed lamina. Once assembled and aligned, the lamina are joined by known methods into a unitary body such as by sintering of uncured ceramic lamina, known as green sheets, or fusing or bonding of thermoplastic lamina.
Such modular circuits may be extremely complex and require hundreds of sequentially performed processes (with testing and repair between at least small groups of steps) to produce. For example, the lamina may be punched to form an array of vias, screening is done to fill vias and form conductive patterns (implying that a production run for each unique pattern will be performed for each required pattern forming a plurality of identical lamina to be eventually be placed in each of a like plurality of modular circuits), a covering to protect the pattern and avoid contamination applied to each lamina, selection and ordering of the lamina, removal of the covering, placement and alignment of the lamina in a stack and joining of the lamina in the stack.
Even prior to screening and assembly of the green sheets, many complex processes have been required. Specifically, green sheet material has been supplied in rolls which is cut by a blanking die to approximate size. At the same time, locating holes are cut into the smaller sheets by the blanking die. The smaller sheets are then partially cured for an extended period of time, perforated with vias, stacked by capture in another punching die and again cut by another blanking die to a desired final size including a margin beyond the dimensions to which the final structure will be diced. The two blanking die cutting operations, particularly the second when green sheets are stacked and trimmed, provide a source of debris from the kerf, causing possible contamination of the green sheets with material which is cut away.
Further and perhaps more importantly, the edge formed by the second die cutting operation has been used as a reference for screening and final stacking registration, transfer to a cold frame having particular dimensions at ambient temperature and further transfer to a warm frame having the same dimensions as the cold frame when at a temperature of 80.degree. C. for lamination whereas the registration holes have been used for punching vias. Since the vias must be accurately aligned in the finished product, any potential variation in alignment between the alignment hole and the second die cutting operation, as well as the use of a cold frame, imposes a manufacturing tolerance which must be accommodated.
It should also be appreciated that the tooling necessary for each of the two punch blanking operations is expensive both to provide and maintain. High precision dies are necessary, particularly for capture and alignment of the green sheets. The punches are fabricated from tungsten carbide to have a sufficiently durable edge but which require difficult and expensive machining to renew. Down time of the punch apparatus for approximately one day is required when the punch must be resurfaced and sharpened. Renewing of punches is required with particular frequency when cutting uncured ceramic green sheets since the ceramic material contains a high percentage of alumina which is highly abrasive. Further, the effectiveness of punches is necessarily limited for cutting stacks of material since a layer may be substantially deformed by cutting force before the punch blade emerges from another layer being cut.
Currently, most of the sequentially performed process steps subsequent to the screening process must be performed manually due to the requirement for extremely high alignment or registration accuracy and only small groups of steps have, to date, been automated. In spite of the substantial expense of such labor intensive processes, the performance of modular circuits often justifies the expense since no reasonable alternatives providing comparable performance and manufacturing flexibility and reliability are available.
The manufacturing process is further complicated by the unavoidable possibility of human error in assembly (e.g. lamina order or alignment during assembly of the lamina stacks). While the possibility of repair (known as an engineering change or, simply, EC) of modules which may fail tests performed during the course of manufacture exists and is also provided for in the design of lamina, a gross error in lamina order or alignment is unlikely to be repairable; adding to the cost of functional modules produced.
Automation of manufacture of modular circuits has also been complicated by the nature of the lamina which are included in the modular circuit. Whether the lamina are thermoplastic polymer, uncured ceramic or other material, the lamina must be kept thin to avoid the bulk and additional conductor length therethrough that would otherwise be required as well as avoiding increased stresses during thermal cycling of the modular package once placed in service. Further, the need to laminate the lamina together implies that the materials cannot be in their final form and must have properties which facilitate the lamination process. Both thinness of the lamina and their required material properties lead to an unavoidable fragility of the lamina.
Moreover, while conductive patterns of screened paste on the lamina can be stabilized somewhat by treatment (e.g. heating, drying, etc.) after screening, the conductive patterns are very delicate and subject to damage due to both the state of the conductive material and the fineness of the patterns, particularly in layers which increase the pitch of patterned features to match connection pads of chips to be mounted thereon. Therefore, shearing motion between lamina must be scrupulously avoided when lamina are in contact with each other.
It has also been the trend in electronic devices that once a new technology has been introduced and experience gained with manufacturing and design of circuitry using that technology, the technology is applied to other more economically accessible devices. The increased performance and functionality provided by the new technology increases demand for application of the technology to an increased variety of products. By the same token, the increased experience with design and manufacturing processes tends to increase manufacturing yield and reduce cost of exploiting the new technology in a wide variety of products. However, both the compromise of the manufacturing yield and labor costs attributable to extensive human intervention in the manufacturing process has prevented sufficient reduction in cost of modular circuits for widespread use of modular circuits at the present time. Further, production volume is far too limited by labor intensive manufacturing methods to satisfy demand which would accompany use of modular circuits in a wide variety of products or even data processors usable as personal computers. Efforts to accomplish some degree of automation of the assembly process has not provided for any significant increase in throughput since all parts of an automated assembly process must be integrated for continuous assembly to increase production.
It can be appreciated from the foregoing that an extremely critical phase of manufacture of modular circuit packages which has proven a major impediment to automation is the alignment and registration of individual lamina as they are assembled into a stack. While the use of alignment pins is known in other processes (e.g. photography) to achieve and maintain high accuracy of registration of lamina, the number of lamina involved is usually very small whereas present designs generally require at least five to nine lamina (although possibly only two) and an increase in the number of lamina to perhaps over one hundred is projected.
Additionally, when increased throughput is developed by automation, it is convenient to provide for stacking of groups of stacked lamina corresponding to complete modules in order to save staging space until lamination processes are carried out. Such groups of stacked lamina must remain in good registration throughout the lamination process to avoid application of shearing forces within the individual stacks and to assure that compressional forces applied during lamination are sufficiently uniform over each stack and throughout a group of stacks.
Thus, to justify the cost outlay for automation and the development of generalized machinery to do so, substantial length (e.g. at least close to one inch) of alignment pins would be required. However, close matching of alignment pin size and location with complementary apertures in lamina is required in order to obtain the required accuracy of registration. This constraint implies friction between the lamina and the alignment pins which can change orientation of the lamina and cause binding. Green sheets, in particular, are quite brittle at room temperature (although "rubbery" at a small temperature increment thereover) and any binding is likely to cause breakage near an alignment hole.
Further, the flexibility of the lamina disturbs the geometry of hole location, as does orientation of the lamina (e.g. diverging from orthogonal to the alignment pins) making binding of the lamina on long pins unavoidable. When binding occurs as the lamina are pressed together, tearing or other damage of the fragile lamina at the alignment apertures has invariably occurred, destroying the capability of such an arrangement to provide accurate registration. At least as importantly, the flexibility of the lamina may permit a wiping motion between lamina as lamina are pressed together which can greatly damage screened paste connections while concealing such damage from detection by visual inspection since damaged connections would be between lamina.